From the EP-A-0 306 307, which herewith is declared an integral part of the present description, it is known, within the framework of the recycling of containers, to detect on empty containers, in particular plastic containers, e.g. plastic bottles, whether any contaminations are present inside the container.
To this end it is proposed to detect such contaminations with the aid of an ionization technique, e.g. flame ionization or photo ionization in the UV-range, and if need be to eliminate the contaminated containers before re-filling them.
From the WO88/00862 a selection method of containers is known, with which either distilled water is sprayed into the containers, water that may now be contaminated is removed from the containers and analysed, or water is sprayed into the containers, shaken in same and then analysed.
To increase the analysis redundancy, it is well known to use two different analysis techniques, The output signal of the detectors of the two analysis techniques is first of all compared with reference data, tho comparison result is then compared with a correlation value range which depends on the specific analysis technique and a specific product which originally was present in the container.
On which basis the deciding correlation factors are determined according to this publication, is not indicated. Described is the use of detectors which specifically detect a given substance, e.g. a sugar analyser, and the providing of two such detectors that respond with a narrow band to a substance would make it possible to detect compound contaminations.
In principle, with regard to the technique described in the WO88/00862, it must be regarded as disadvantageous that liquid samples must be taken from the containers in question, so that the test method based thereon is extremely slow. On the other hand, because of the slowness resulting from the use of a liquid, there is sufficient time to test the liquid sample for contaminations under conditions that are practically similar to those in the laboratory.